Lung infections caused by bacteria, Pseudomonas aeruginosa are the main cause of recurrent illness and lung damage in children with CF. Are aims are to define the cumulative incidence of upper and lower airway colonization with P. aeruginosa in patients with CF within the first 3 years, to determine when P. aeruginosa infection occurs as defined by serologic response. and to characterize each lower airway P. aeruginosa and compare to those in the oropharynx.